Many communication networks that provide high bit-rate transport over a shared medium are characterized by non-continuous, or burst mode, data transmission in the upstream direction and continuous data transmission in the downstream direction. An example of such a network is a passive optical network (PON) 100 schematically shown in FIG. 1. A typical PON 100 includes a plurality of optical network units (ONUs) 120-1 through 120-M coupled to an optical line terminal (OLT) 130 via a passive optical splitter 140. Since all ONUs function in like manner, they will be collectively referred to by the reference numeral 120 in the following description unless reference is made to a specific ONU.
Traffic data transmission is performed over two optical wavelengths, one for the downstream direction and another for the upstream direction. Thus, downstream transmission from the OLT 130 is broadcast to all the ONUs 120. Each ONU 120 filters its respective data according to, for example, pre-assigned labels. Transmission from an ONU 120 to the OLT 130 is in the form of a burst. The OLT 130 continuously transmits downstream data to the ONUs 120 and receives upstream burst data sent to OLT 130 from ONUs 120. The OLT 130 broadcasts data to the ONUs 120 along a common channel so that all the ONUs 120 receive the same data. An ONU 120 transmits data to the OLT 130 during different time slots allocated by the OLT 130. That is, the OLT 130 is aware of the exact arrival time of data and the identity of a transmitting ONU 120.
A PON is typically designed with varied lengths of optical links, splits, cost driven optics, and other physical consideration. Thus a typical PON suffers from optical aberrations influencing the signals. Therefore, appropriate signal processing is required in order to recover the original signal from the received signal and to avoid errors during transmission.
An optical signal sent from an OLT 130 is received by a receiver in the ONU 120 and converted into an analog electrical signal. The ONU's receiver uses a clock and data recovery (CDR) circuit to generate a clock corresponding to the incoming data, thereby correctly retiming the digital incoming data. After recovering the data, a forward error correction mechanism may be utilized to detect and correct errors in the received data and to provide an assessment of the signal quality. However, during the recovery process, essential information, such as eye distortion, frequency movement, phase information, jitter, and other effects are discarded, and thus the quality of the input signal cannot be correctly measured. Therefore, assessment of the signal quality is necessary prior to recovering the signals.
In PON systems there is an increasing demand to perform an optical line analysis to determine the root cause of the PON failures or performance degradation. Results of an optical line analysis can enable PON operators to perform optical layer supervision. The optical supervision allows more efficient operation and maintenance of PON networks, for example, by not sending technicians if the PON system works properly, dispatching the correct technician if a problem is detected, or providing correct diagnostics to the technician.
Optical line analysis of signals can be performed only on signals that are not fully recovered and cannot be performed using conventional techniques for detecting the errors in the received signals.
Therefore, it would be advantageous to provide a solution for performing an optical line analysis in passive optical networks.